II-VI semiconductor light emitting device and its manufacturing method

Abstract: In a semiconductor light emitting device of a group II-V semiconductor, a current shutting-off layer (6, 16) for passing the current only through a belt-shaped central area (6a) is formed in one of the two light shutting-up layers (5, 7, 15, 17) sandwiching an active layer (4, 14), so that the light emission efficiency improves and a light guiding path is provided. In a process for forming each layer on a substrate through epitaxial growth, the belt-shaped central area (6a) is formed by etching the current shutting-off layer (6, 16).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor light emitting device and its manufacturing method, said light emitting device being suitable for use as a semiconductor laser device, as a signal reading and writing light emitting device for use in a compact disk (CD) player and a laser disk (LD) player, as a light emitting device for use in a bar code reader, and as a blue light emitting device (LED) for use in displays of other various electronic apparatuses. Description of the Prior Art Fig. 1 shows a blue light emitting semiconductor layer device of ZnSe as an example of a conventional semiconductor light emitting device. In the semiconductor laser device shown in this figure, a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe is grown on an N-type GaAs substrate 21, and a buffer layer 22 made of N-type ZnSe, a light shutting-up layer 23 made of N-type ZnSSe, an active layer 24 made of ZnCdSe, a light shutting-up layer 25 made of P-type ZnSSe and an electrode contact layer 26 made of P-type ZnSe are formed in this order in lamination. On the electrode contact layer 26 which is the top layer, a metal such as Au is directly deposited to form an electrode 27. Similarly, an electrode 28 is formed on the outer surface of the N-type substrate 21. The light shutting-up layers 23 and 25 made of N-type and P-type ZnSSe, respectively, function to prevent the diffusion of light caused at the active layer 24. In an device having such a P-N junction structure of the group II-VI semiconductor, when a bias voltage is applied between the electrodes 27 and 28 in a forward direction, carriers injected by the current are shut up in the active layer 24, so that an induced emission occurs vigorously. When the exciting current exceeds a threshold value, light resonates between the parallel end surfaces of the active layer 24 to cause a laser oscillation. In this case, to improve the output performance of the device, it is important to increase the ratio of the current which contributes to the light emission. That is, since the light emitting area of the active layer 24 is limited to a belt-shaped central light emitting area 24a, to increase the ratio of the current which contributes to the light emission, it is desirable to limit the spread of the current in a horizontal direction (direction parallel to the active layer 24) to shut up as much current as possible in the central light emitting area 24a of the active layer 24. In a conventional device, the current is shut up in the following manner: In forming the electrode 27 on the P-type buffer layer 26 which is the electrode contact layer, after a metal is deposited, only a belt-shaped area located opposite the central light emitting area 24a of the active layer 24 is left as the electrode 27 and the other portions are removed by etching, thereby restricting the horizontal spread of the current flowing from the electrode 27 to the active layer 24. In this structure, however, since there is a considerable distance between the electrode 27 and the active layer 24 because of the presence of the light shutting-up layer 25 and the electrode contact layer 26 therebetween, the current spreads as shown by the arrows L' before it reaches the active layer 24. As a result, the current also reaches the outside of the central light emitting area 24a of the active layer 24. For this reason, the ratio of the current which flows through the central light emitting area 24a of the active layer 24 to contribute to the light emission is largely reduced. Consequently, to obtain a necessary current density, a high voltage has to be applied between the electrodes. This not only increases the power consumption but also increases the generation of heat. As a result, the temperature characteristic receives a bad influence. To restrain the spread of the current to solve these problems, the distance between the electrode 27 and the active layer 24 may be decreased by reducing the thickness of the P-type clad layer (light shutting-up layer) 25 and the P-type buffer layer (electrode contact layer) 26 as much as possible. However, in actuality, since the thicknesses of the P-type layers 25 and 26 have to be at least 1µm to decrease the influence by the absorption and reflection of light caused at the active layer 24, it is difficult to decrease the distance between the electrode 27 and the active layer 24 under present circumstances. The light shutting-up layer 25 shuts up the light travelling in a vertical direction from the electrode 27 toward the active layer 24. However, in the horizontal direction, there is no difference in refractive index and light absorption since there is only one layer made of only one component, i.e. P-type ZnSSe. Therefore, there is substantially no light guiding path in the horizonal direction, and the guiding of the light in the horizontal direction is made only by shutting up the current. For this reason, to control the angle of spread of light, there is no other way than regulating the distribution of spread of the current. In such a way, however, it is difficult to minutely control the angle of spread of light. A III-V laser structure comprising a n-type GaAs current blocking layer within the p-type cladding region is known from article by A.O.Harm published in Philips Journal of Research, Vol.45, No.3/4, p.177-187 (1990). SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor light emitting device and its manufacturing method. In the semiconductor light emitting device, the power consumption is reduced and the temperature characteristic is improved by increasing the ratio of the current which contributes to the light emission by providing in the light shutting-up layer a structure for preventing the spread of the current, and the light guiding path in the horizontal direction in the light shutting-up layer is minutely controlled. The above-mentioned object is achieved by a semiconductor light emitting device according to claim 1. According to this structure, the current flowing between the electrodes through the active layer flows, at the layer level where the GaAs current shutting-off layer is formed, only through the belt-shaped central area where no current shutting-off layer is formed. Therefore, the current flows in concentration through the central light emitting area of the active layer which is located opposite the belt-shaped central area mentioned above. Further, since the GaAs current shutting-off layer and the active layer are much closer to each other than in the conventional device, the spread of the current is restrained greatly, so that most of the current contributes the light emission. Further, since the GaAs constituting the current shutting-off layers and the group II-VI semiconductor constituting the second and third light shutting-up layers are different in refractive index, the belt-shaped central area between the current shutting-off layers functions as a light guiding path in the horizontal direction because of the difference in light absorption therebetween. As a result, the spread of light in the horizontal direction is minutely controlled by adjusting the width of the belt-shaped central area. The semiconductor light emitting device according to the present invention is manufactured by the method defined in claim 2. By removing the belt-shaped central area located opposite the central light emitting area of the active layer in the GaAs layer by etching, the GaAs current shutting-off layer is formed on each side of the belt-shaped area. Then, the group II-VI semiconductor is grown on the surfaces of the GaAs current shutting-off layers and on a portion of the second light shutting-up layer which is exposed to the belt-shaped central area, thereby forming the third light shutting-up layer of the same conductivity type as the conductivity type of the second light shutting-up layer. Preferably, the growth of the GaAs current shutting-off layer is performed at a substrate temperature lower than a substrate temperature at which the group II-VI semiconductor is grown. BRIEF DESCRIPTION OF THE DRAWINGS This and other objects and features of this invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: Fig. 1 is a cross-sectional view schematically showing the structure of a conventional semiconductor laser device; Fig. 2 is a cross-sectional view schematically showing a first embodiment of the present invention; Fig. 3 is a perspective view showing a relevant portion of the first embodiment; Fig. 4 graphically shows the quantity of light absorption in a belt-shaped central area and in N-type GaAs current shutting-off layers of the first embodiment; Fig. 5 is a cross-sectional view schematically showing a condition where the GaAs current shutting-off layer is formed in the manufacturing process of the first embodiment; Fig. 6 is a cross-sectional view schematically showing a condition where the belt-shaped central area is formed by etching the GaAs current shutting-off layer in the manufacturing process of the first embodiment; Fig. 7 is a cross-sectional view schematically showing a condition where a third light shutting-up layer is formed after the etching of the belt-shaped central area in the manufacturing process of the first embodiment; Fig. 8 graphically shows the etching speeds of GaAs and ZnSSe; and Fig. 9 is a cross-sectional view schematically showing the structure of a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments where the present invention is employed in a semiconductor laser device will be described with reference to the drawings. Fig. 2 schematically shows a first embodiment of the present invention. The device shown in this figure is a blue light emitting semiconductor laser where a P-N junction structure including a plurality of layers is formed on an N-type GaAs substrate 1 by epitaxially growing a group II-VI semiconductor of ZnCdSSe. Specifically, a buffer layer 2 made of N-type ZnSe, a first light shutting-up layer 3 made of N-type ZnSSe, an active layer 4 made of ZnCdSe and a second light shutting-up layer 5 made of P-type ZnSSe are formed on the N-type GaAs substrate 1 in this order. On the second light shutting-up layer 5, a third light shutting-up layer 7 is formed with subsequently-described N-type GaAs current shutting-off layers 6 between. On the third light shutting-up layer 7, an electrode contact layer 8 is formed. On the entire outer surfaces of the electrode contact layer 8 and the N-type GaAs substrate 1, electrodes 9 and 10 are formed, respectively, by depositing a metal such as Au. The third light shutting-up layer 7 which is made of P-type ZnSSe like the second light shutting-up layer 5 constitutes a P-type clad layer together with the second light shutting-up layer 5. Likewise, the first light shutting-up layer 3 constitutes an N-type clad layer. The electrode contact layer 8 made of P-type ZnSe constitutes a buffer layer. In the semiconductor laser device having the above-described structure, when a bias voltage is applied between the electrodes 9 and 10 in a forward direction, the current flows from the electrode 9 to the active layer 4 by way of the electrode contact layer 8, the third light shutting-up layer 7 and the second light shutting-up layer 5. This current causes holes to flow from the second and third light shutting-up layers 5 and 7 constituting the P-type semiconductor layer to the active layer 4, and causes electrons to flow from the first light shutting-up layer 3 constituting the N-type semiconductor layer to the active layer 4. By the injection of the carriers, the recombination occurs of the electrons and holes which are shut up in the active layer 4 where the energy level is low, so that natural light is emitted from a central light emitting area 4a of the active layer 4. When the exciting current exceeds a threshold value, the light emission of the active layer 4 is changed from the natural light emission to an induced emission, so that light resonates between the parallel end surfaces of the active layer 4 to cause a laser oscillation. Fig. 3 shows the upper half of the device where the N-type GaAs current shutting-off layer 6 is formed. As shown in Figs. 2 and 3, the N-type GaAs current shutting-off layer 6 is formed between the second light shutting-up layer 5 and the third light shutting-up layer 7 on each side of the surface of the second light shutting-up layer 5 so that a belt-shaped central area 6a is left. The belt-shaped central area 6a is located opposite the central light emitting area 4a. The third light shutting-up layer 7 is integrated with the second light shutting-up layer 5 through the belt-shaped central area 6a. A thickness t 1 of the second light shutting-up layer 5 is set to be smaller than one third of a thickness t o of the whole light shutting-up layer constituted by the second and third light shutting-up layers 5 and 7, so that the current shutting-off layers 6 and the belt-shaped central area 6a are as close to the active layer 4 as possible. Specifically, the thickness t 0 of the whole light shutting-up layer is approximately 1.5µm, whereas the thickness t 1 of the second light shutting-up layer 5 is less than 0.5µm. In this structure, the current flows from the electrode 9 to the electrode 10 by way of the belt-shaped central area 6a, as shown by the arrows L in Fig. 2. The N-type GaAs shutting-off layer 6 prevents the current from spreading in the horizontal direction. Therefore, the current flows through the central light emitting area 4a of the active layer 4 in concentration. Fig. 4 shows the quantity of light absorption in the belt-shaped central area 6a and in the N-type GaAs current shutting-off layers 6. As shown in the figure, since the quantity of light absorption is different between the N-type GaAs current shutting-off layers 6 and the second and third light shutting-up layers 5 and 7 constituting the P-type ZnSe layer because of the difference in refractive index therebetween, the belt-shaped central area 6a forms a trough of light absorbance to function as a light guiding path in the horizontal direction. Therefore, the spread of light in the horizontal direction can be controlled by adjusting a width B of the belt-shaped central area 6a. Figs. 5, 6 and 7 sequentially show an example of a process for manufacturing the device in this embodiment. First, the group II-VI semiconductor is epitaxially grown on the N-type GaAs substrate 1 at a predetermined substrate temperature (e.g. 350°C) to form the N-type buffer layer 2, the N-type first light shutting-up layer 3, the active layer 4 and the thin P-type second light shutting-up layer 5 in this order in lamination. Then, N-type GaAs is epitaxially grown on the entire surface of the second light shutting-up layer 5 to form an N-type GaAs layer 6'. The growth of the N-type GaAs layer 6' is performed at a substrate temperature lower than the substrate temperature at which the group II-VI semiconductor is grown. By growing the N-type GaAs layer 6' at a lower substrate temperature, the N-type GaAs layer 6' is prevented from being diffused into the group II-VI semiconductor film to form a highly electrically resistant alloy layer. As a result, the deterioration of the group II-VI semiconductor film is prevented. Then, as shown in Fig. 6, after forming the pattern of the N-type GaAs current shutting-off layers 6 by photolithography, only an area 6a' (hatched portion in the figure) in the N-type GaAs layer 6' corresponding to the belt-shaped central area 6a is etched to form the N-type GaAs current shutting-off layer 6 on each side of the belt-shaped central area 6a. Fig. 8 shows a relationship between the etching time and the etched quantity of GaAs and ZnSSe when an ammonia, hydrogen peroxide etchant is used. As shown in the figure, ZnSSe which is a group II-VI semiconductor constituting the second light shutting-up layer 5 is hardly eroded in four minutes, whereas GaAs constituting the current shutting-off layers 6 is effectively eroded in proportion to the etching time; it is eroded by approximately 1µm in four minutes. Thus, the ammonia, hydrogen peroxide etchant is suitable for removing a part of the N-type GaAs layer 6' grown on the group II-VI semiconductor layer since only GaAs is etched by regulating the etching time because of the large difference in etching speed between GaAs and group II-VI semiconductor. After the N-type current shutting-off layers 6 are formed in this manner, as shown in Fig. 7, P-type ZnSSe which is a group II-VI semiconductor is epitaxially grown on the current shutting-off layer 6 and on a portion of the second light shutting-up layer 5 which is exposed to the belt-shaped central area 6a, thereby forming the third light shutting-up layer 7 which is integrated with the second light shutting-up layer 5 at the belt-shaped central area 6a. Then, on the third light shutting-up layer 7, P-type ZnSe which is a group II-VI semiconductor is epitaxially grown to form the electrode contact layer 8. The growth of the third light shutting-up layer 7 and the electrode contact layer 8 can be performed at a substrate temperature similar to the substrate temperature at which the second light shutting-up layer 5 is formed. In this embodiment, since the current is concentrated on the central light emitting area 4a of the active layer 4 because of the presence of the current shutting-off layers 6, no problem is caused even if the electrode 9 is formed on the entire surface of the electrode contact layer 8. Fig. 9 shows as a second embodiment a semiconductor laser device where a P-N junction structure is grown on a P-type GaAs substrate 11. Specifically, as shown in Fig. 9, a buffer layer 12 made of P-type ZnSe, a first light shutting-up layer 13 made of P-type ZnSSe, an active layer 14 made of ZnCdSe and a second light shutting-up layer 15 made of N-type ZnSSe are formed in this order on the P-type GaAs substrate 11. On the second light shutting-up layer 15, a third light shutting-up layer 17 made of N-type ZnSSe is formed with P-type GaAs current shutting-off layers 16 between. On the third light shutting-up layer 17, an electrode contact layer 18 made of N-type ZnSe is formed. Reference numerals 19 and 20 represent electrodes formed on the outer surfaces of the electrode contact layer 18 and the P-type GaAs substrate 11, respectively. As described above, the present invention may be employed in a semiconductor laser device where a group II-VI semiconductor film is grown on the P-type GaAs substrate 11. In this case, the current shutting-off layers 16 are made of P-type GaAs in contrast to the N-type light shutting-up layers 15 and 17. While the group II-VI semiconductor film is made of a semiconductor of ZnCdSSe in the above-described embodiments, the same workings and advantages are obtained when the semiconductor film is made of a semiconductor of MgZnCdSSe. As described above, according to the present invention, the current flowing between the electrodes are concentrated on the central light emitting area of the active layer. Further, since the GaAs current shutting-off layers and the active layer are close to each other, most of the current is made to contribute to the light emission by effectively restraining the spread of the current. Thereby, the emission efficiency remarkably increases to reduce the power consumption. Since the generation of heat from the device due to excessive leakage current is largely reduced, an excellent temperature characteristic is realized. Since the belt-shaped central area between the current shutting-off layers functions as a light guiding path in the horizontal direction, the guiding of the light is easily controlled only by adjusting the width of the belt-shaped central area. As a result, a spread characteristic of light necessary for the device is readily and easily realized. In addition, since the current is concentrated on the central light emitting area of the active layer because of the presence of the current shutting-off layers, the electrode may be formed on the entire surface of the group II-VI electrode contact layer. In this case, since the electrode contact layer and the electrode are in contact with each other at a larger area, less resistance is generated there. As a result, the generation of heat from the device is further reduced. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.

Claims: A semiconductor light emitting device comprising: a substrate (1, 11) made of GaAs; a first light shutting-up layer (3, 13) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a same conductivity type as a conductivity type of the substrate; an active layer (4, 14) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe, said active layer being formed on the first light shutting-up layer;; a second light shutting-up layer (5, 15) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a different conductivity type from the conductivity type of the first light shutting-up layer, said second light shutting-up layer being formed on the active layer; a current shutting-off layer (6, 16) made of GaAs of a different conductivity type from the conductivity type of the second light shutting-up layer, said current shutting-off layer being formed on the second light shutting-up layer and a belt-shaped central area (6a) thereof being removed by photolithography and etching so as to pass a current therethrough; and a third light shutting-up layer (7, 17) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a same conductivity type as the conductivity type of the second light shutting-up layer, said third light shutting-up layer being formed on the current shutting-off layer and on a portion of the second light shutting-up layer which is exposed to the belt-shaped central area of the current shutting-off layer. A method of manufacturing a light emitting semiconductor device, comprising: a first step in which, on a substrate (1, 11) made of GaAs, a first light shutting-up layer (3, 13) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a same conductivity type as a conductivity type of the substrate, an active layer (4, 14) made of ,and a second light shutting-up layer (5, 15) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a different conductivity type from the conductivity type of the first light shutting-up layer are formed in this order; a second step in which a current shutting-off layer (6, 16) made of GaAs of a different conductivity type from the conductivity type of the second light shutting-up layer is formed on an entire surface of the second light shutting-up layer; a third step in which a belt-shaped central area (6a) of the current shutting-of layer is removed by etching after a pattern is formed by photolithography; and a fourth step in which a third light shutting-up layer (7, 17) made of a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe of a same conductivity type as the conductivity type of the second light shutting-up layer is formed on the current shutting-off layer and on a portion of the second light shutting-up layer which is exposed to the belt-shaped central area of the current shutting-off layer. A method of manufacturing a light emitting semiconductor device according to claim 2, wherein an ammonia-hydrogen peroxide etchant is employed for the etching in the third step.

Inventors: SHAKUDA YUKIO, SHAKUDA, YUKIO, SHAKUDA, YUKIO, C/O ROHM CO., LTD.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1994
Main classification: H01S5
IPC codes: H01S5, H01L33